Sortation according to given Tolerances

High quality Sealing Technology with O-rings: with more precise Tolerances even more efficient

The O-ring has provided evidence to be an efficient and cheap sealing element in many ways, for example as secondary seal in a floating ring seal of the feed water pump of a nuclear power plant or as seal at a temporary screw-in connection for air conditioning systems in cars. There are a lot more examples like these where O-rings have met the requirements of highest demands on leak tightness and this in different media and at different temperatures from -60 °C to 300 °C. O-rings are readily available to the market, easily mounted and require only simple seal housing. Hence it is especially annoying for design engineers if O-rings have to be discarded as sealing elements for problem solving because there is too much variance in their measurements. To fall back on other seals will generally be more expensive for the user.
Particularly when smaller O-rings are applied the common tolerances according to DIN 3771 part 1 constitute a considerable constraint regarding their applicability. This paper shows reasons for variances in the measurements of O-rings due to the manufacturing process and portrays the resulting effects on function. It describes a new and highly accurate optical measurement process for O-rings which is re-offered as service for the measurement and sortation according to precise tolerances.

How are the Dimensional Fluctuations of O-rings caused?

There are three influencing factors mainly due to the manufacturing process:

• Fluctuations of mould temperatures during the curing of O-rings
• Variances in shrinking behaviour of rubber and other compound ingredients
• Temperature fluctuations can lead to significant variations in diameter dimensions, particularly when manufacturing with large moulds within the same delivery batch. However, even with very well temperature-controlled processes, such as those possible with small injection moulds, batch-dependent variations in the mixture and slight batch-dependent temperature differences in the mould can lead to clearly measurable fluctuations in the inner diameters of O-rings.

Fluctuations of temperatures can lead to massive variances of diameter measurements within the same batch especially if O-rings are manufactured with big moulds. Even during highly temperature controlled processes like the ones which are possible with small injection moulds, clearly measurable fluctuations of inner diameters can occur due to batch related variances of compound and small batch related temperature differences.
For the generally more important functional dimension of O-rings, the cord thickness, there is a further essential influencing factor, namely the mould offset. Figure 1 illustrates through a simple top view on an O-ring (upper mould half = red; lower mould half = turquoise) how due to the offset of upper and lower half variances of cord thickness can occur on an O-ring. While this offset is normally smaller than 0,05 mm on new injection moulds, it can increase significantly due to abrasion and may add up to 0,08 to 0,15 mm according to DIN 3771 part 4.

The three influencing factors described above ultimately led to the dimensional tolerances specified in DIN 3771/Part 1 (Figure 2), which most O-ring manufacturers also use for their products. It is difficult to find suppliers of O-rings that significantly restrict these tolerances (and also adhere to them). This would be particularly useful for small O-rings, as these are considered to have too generous tolerances from the user’s point of view. A comparison of two standard sizes illustrates this. The O-ring with dimensions (inner diameter x cord thickness) 2.0+/-0.13 × 1.80 +/- 0.08 has significantly less favourable tolerances compared to the O-ring 20+/-0.22 × 3.55+/-0.1. While these are +/-6.5% of the inner diameter and +/-4.4% of the cord thickness for the small O-ring, they are significantly more favourable for the larger O-ring at +/-1.1% of the inner diameter and +/-2.8% of the cord thickness. This ratio becomes even less favourable when using smaller cord thicknesses, for example 1 mm, as the manufacturer’s tolerances do not decrease significantly.

The Influence of Variances in Cord Thickness and Inner Diameter Tolerances on the Functional Behaviour of O-ring Seals

The seal effect of O-rings is based on generating surface pressure through the deformation of the O-ring cord thickness. As rubber materials do not behave ideally elastic, O-rings need a minimum deformation of approx. 6 % of the cord thickness. When the deformation remains under this value, the compression will soon be 100 % especially regarding small cord thicknesses. Typical values for the compression of cord thickness range from 20% to 29% for a radial sealing static sealant and from 12% to 22% for a dynamic sealant [1], each related to an O-ring cord thickness of 1,78 mm with a nominal diameter of 20 mm and a concentric position of the components which are to be sealed. Considering also the maximal possible eccentricity, the range of variation can get even wider depending on the size of the diameter clearance.

As the reaction forces of a deformed O-ring proceed progressively over the relative deformation [2], see figure 3, this means a proportion of maximal to minimal deformation respectively friction force of ca. 3:1 for the dynamic sealant in the example above. Because of this possible variance O-rings are discarded as sealing solutions for many dynamic applications due to the massive tolerances. For static O-ring applications these typical variances of deformation forces mean sealing face pressures which are dependent on tolerances that have a substantial influence on the leakage rate of O-ring connections [3]. However, because statically applied O-rings generally are deformed much more as dynamically applied O-rings, this effect does not constitute an untenable constraint.

Realisable Tolerances for small O-rings

The progress of technology has clearly not stopped at the production of small O-rings. Be it the precision of the production of moulding tools or their rheological dimensioning by calculation programmes, be it the temperature control of injection machines by use of microprocessors or by applying refined deflashing procedures, all of this makes it possible that in the meantime a very narrow normal distribution regarding functional dimensions is feasible, see figure 6 and 7. A check on customary NBR-O-rings of different suppliers (inner diameter < 20 mm) showed that more than 90 % of the tested O-rings complied with clearly narrower tolerances. Useful tolerances, meaning that over 90 % of the parts shall comply with, can be at an average of radial cord thickness of +/- 0,03 mm and an average of inner diameter of +/- 0,05 mm regarding nominal size. This would of course imply that the injection mould is dimensioned exactly for the desired mean and the used rubber material and that the maximal permitted value for the mould offset according to DIN 3771 is not fully used. Through this narrowing of tolerances the possible range of variation of the derformation of the cord thickness can be limited considerably und thereby the field of application of this O-rings especially for dynamic applications can be expanded significantly.

Measuring and Sorting as Service

If you would like to sort a batch of O-rings according to specified tolerances, please contact us. Due to the effort involved in setting up the automatic sorting system, automated testing only makes sense for larger quantities of around 5,000 to 10,000 parts or more.

References

[1] inPHorm, PC-Programme for the calculation and selection of O-rings, Parker Dichtungen [2] Dale Patterson, More Tips for Seal Design: Effects of Cross-Section Compressive Forces, company publication Greene Tweed [3] Klaus Scheerer, Einfluss der mechanischen Eigenschaften von O-Ringen auf deren Verhalten in Miniatur-Dichtverbindungen, Dissertation Universität Stuttgart